1. Field of the Invention
The invention is related to the field of test systems, and in particular, to a gas test system for calibrating units under test.
2. Statement of the Problem
Flow meter designers often need to test new designs of flow meters for accuracy, quality, etc. Most design companies are able to do in-house testing of flow meters using liquids, as the test facilities for liquids can be set up without undue expense or time. Test facilities for gas are not as common in-house. Thus, the designers usually have to travel to a test facility and pay to use the test facility. This can be costly and time consuming. Plus, the designers would benefit from immediate feedback on design changes. As it sits now, the designers may have to wait weeks or months before they can test new designs and get test results on the new designs. The designers may benefit from a test facility for gas that is small enough and cost effective enough to set up in-house.
There are two main types of test facilities for gas. One type is a blow-down system. In a blow-down system, a compressor takes air at atmosphere and compresses the air into a tank. When the pressure in the tank is at a desired pressure for testing, the air is released from the tank and passes through a reference meter and a unit under test (UUT). The air is then vented back to atmosphere. The reference meter and the UUT measure the flow rate of the gas as it travels from the tank back to atmosphere. The measurements from the reference meter are used to calibrate the UUT. Unfortunately, blow-down systems have short runtimes, are costly and inefficient, and are extremely noisy. A blow-down system is illustrated in FIG. 1 and discussed further below.
Another type of test facility is a re-circulating gas loop. The Metering Research Facility operated by Southwest Research Institute® of San Antonio, Tex. uses a re-circulating gas loop. The Metering Research Facility includes the gas loop, a compressor, a chiller, sonic nozzles, and stations for units under test (UUT). The compressor circulates the gas around the gas loop at a desired flow rate. The compressor adds heat to the gas in the gas loop when it circulates the gas. The chiller cools the gas in the gas loop to a desired temperature. The UUT and one or more sonic nozzles measure the flow rate of the gas. The sonic nozzles are the reference meters for the UUT. The measurements from the UUT are compared to the measurements from the sonic nozzles to verify the accuracy of the meter under test or calibrate the UUT. Unfortunately, the Metering Research Facility is very large in size, is costly, and requires a lot of power to operate. Current Metering Research Facilities cannot be effectively assembled and operated in many companies due to size, cost, and power requirements. A Metering Research Facility is illustrated in FIG. 2 and discussed further below.